An ongoing challenge for the OLED lighting industry is to improve the efficacy of OLED lighting panels through improved light extraction. At the present time, OLED lighting panel efficacy is limited by light loss through waveguiding in the OLED stack. For example, in a typical planar bottom-emission OLED with no additional light extraction fixtures only as little as 20% of the generated photons can escape from the OLED stack and be emitted as useful illumination. The remaining photons are waveguided in the OLED stack.
For example, FIG. 1 depicts a simplified ray diagram of a planar OLED demonstrating light loss through substrate and anode/organic layer waveguiding. In general, only a small fraction of light emitted relatively close to the surface normal (<θC) is outcoupled.
As shown in FIG. 1, light generated in an OLED can be classified into three radiative modes: the outcoupled mode, the substrate waveguided mode and the anode/organic waveguided mode. Depending on the angle of emission θ, photons are outcoupled as useful emission, or waveguided into the substrate or anode/organic layers. The only outcoupled modes are those with angle to the surface normal less than the critical angle θC, where θC=sin−1(nair/norg)≈36° for norg=1.7. Substrate modes are emitted through the edges of the substrate, whereas anode/organic modes are heavily attenuated by self-absorption, and do not emit through the edge.
Some improvement in light extraction efficiency may be obtained through the use of a light extraction block. The light extraction block may be optically connected to the OLED substrate emissive surface. An example of such an approach is detailed in U.S. Pat. No. 6,952,079 to Shiang et al. (hereinafter “Shiang”). According to Shiang, a “luminaire” with a planar emitting surface may be provided with a recess to receive an OLED device. The luminaire includes sharply angled sides that extend at a relatively shallow acute angle, and are coated with a highly reflective material. Thus, the acutely angled sides and reflective material direct light towards the planar front surface of the luminaire.
An alternative approach is described in D'Andrade & Brown, APL (2006) (hereinafter “D'Andrade”), which describes an OLED device with a truncated square-pyramid luminaire.
Although the above techniques have proven to be somewhat more effective in terms of light extraction enhancement, they still suffer from a number of shortfalls that limit their efficiency and utility. For example, configurations such as Shiang and D'Andrade do not have optimized geometry for maximum light extraction efficiency or for spreading light away from the surface normal. In this regard, Shiang is limited to an illumination from the front surface that lies parallel to the rear surface, and does not enhance any illumination away from the front surface normal. D'Andrade also suffers from drawbacks in terms of uniform emission intensity and color with viewing angle, and exhibits a strong color shift with viewing angle, depending on which surface light is emitted from.
For a point source, the optimized geometry for minimizing total internal reflection is a hemisphere, as described in Reineke et al. Nature, vol. 459, pg. 234 (2009), and shown in FIG. 2. Using this approach, for a point source all light rays are approximately normal to the surface, and so can be emitted without internal reflection. This approach, however, has its own drawbacks related to relative volume required for the half sphere, and, in particular, is less effective for large area light sources than it is for point sources.
In view of the foregoing, there are continuing needs for improved means for improving the efficiency and utility of OLED light sources, including, for example, means for providing improved light extraction enhancement, as well as spreading light away from the surface normal of OLED light sources and the like.